Modern wind turbines are commonly used to supply electricity into the electrical grid. Wind turbines of this kind generally comprise a tower and a rotor arranged on the tower. The rotor, which typically comprises a hub and a plurality of blades, is set into rotation under the influence of the wind on the blades. Said rotation generates a torque that is normally transmitted through a rotor shaft, either directly or through the use of a gearbox, to a generator. This way, the generator produces electricity which is supplied into the electrical grid.
Most wind turbines comprise a yaw system used for orienting the rotor of the wind turbine in the prevailing wind direction. Normally, when the rotor is aligned with the wind direction, the yaw system maintains the position by means of brakes (e.g. hydraulic brake callipers and/or electro-brakes of the yaw motors). When the rotor is misaligned from the wind direction the yaw system rotates the nacelle to reach an appropriate alignment with the wind.
The yaw system normally performs this rotation of the nacelle by means of a yaw drive that includes a plurality of (electric or hydraulic) motors with suitable gearboxes for driving gears (pinions) that mesh with an annular gear attached to the nacelle or to the wind turbine tower. The nacelle can thus be rotated around the tower's longitudinal axis in or out of the wind direction. The rotatable connection between the wind turbine tower and the nacelle is called a yaw bearing. The yaw bearing can be of the roller or gliding type and is generally able to handle very high loads.
In order to stabilize the yaw bearing against rotation a means for braking is often necessary.
Such means for braking are known in the prior art. FIG. 1 shows a typical yaw brake system. The yaw brake of FIG. 1 comprises a circular slew bearing 150 (i.e. a yaw bearing) for rotatably mounting the nacelle on the wind turbine tower; an outer ring 130 of the slew bearing is connected with a main frame 160 of the nacelle, and an inner ring 140 is connected to the tower. A hydraulically actuated disk brake may be used to slow rotation of the outer ring. The disk brake typically requires a flat circular brake disk, such as brake disk 120, and a plurality of brake callipers, such as brake callipers 110, with hydraulic pistons and brake pads. The brake disc 120 may be connected to the inner ring 140 of the slew bearing. Brake callipers 110, attached to main frame 160 may engage with brake disc 120 to brake the inner ring 130.
A plurality of yaw drives (not shown) may be provided. The yaw drives comprise a motor, a gearbox (sometimes referred to as a reducer or as reduction gearing), and a pinion which is arranged to engage with annular gear 170. The hydraulic yaw brakes are able to fix the nacelle in position thus relieving the motors from that task. However, the electro-brakes of the yaw drives typically serve to further brake the yaw system.
Considering the azimuth wind direction and the rotor direction, a wind turbine generally operates as follows:                When the wind turbine rotor is aligned with the wind direction it maintains the position usually by means of the yaw brake system involving hydraulic brake callipers and possibly the electro-brakes of the yaw motors.        When the wind turbine rotor is not aligned with the wind direction, the yaw system rotates the nacelle to reach an appropriate alignment with the wind by means of the yaw drives (yaw motor with gearbox and pinion).        
In the majority of conventional wind turbines, the hydraulic brake callipers are dimensioned to resist approximately 20% of the maximum aerodynamic load torque. The rest is provided by the motors' electro-brakes.
The main technical problem with this type of solution is that the brake callipers' linings don't have a constant friction coefficient over time. The friction coefficient may be affected e.g. due to wear, temperature, brake disc conditions and undesired contamination (oil or grease). If the friction coefficient increases it may cause a premature failure to the brake callipers themselves. On the other hand, if the friction coefficient decreases it may encumber the gearbox motors and in the worst case it may wear down the annular gear.
Furthermore, brake discs require frequent maintenance which increases the operational cost of the wind turbine. Additionally, the yaw brake may further require electro-brakes in the gearboxes to resist the over torques. This may cause unpredictable damages in the gearboxes. Finally, contamination of the linings may not be avoided completely.